The invention relates generally to semiconducting materials and more particularly relates to the doping of semiconducting materials that are grown with Molecular Beam Epitaxial methods.
Laser diodes used as light sources in fiber optic communication systems should produce light with a wavelength in the low loss windows for silica based fibers at approximately 1.3 and 1.55 xcexcm. Production of light with these wavelengths requires quaternary compounds comprising Group III and Group V elements; these compounds are commonly called III-V""s. Two such compounds are GaInAsP and AlGaInAs. The latter of these, AlGaInAs, is of interest since lasers made from this material have decreased temperature sensitivity during operation, allowing them to operate without the need for expensive cooling equipment.
There are two common groups of fabrication methods used to grow III-V compounds. The first group of methods are based on Chemical Vapor Deposition (CVD). Of the various CVD methods Metal-Organic (MO) CVD or Organometallic Vapor Phase Epitaxy (OMVPE) is the principal one. The organic compounds that contain the Group III elements decompose and react on a heated substrate in the presence of gaseous Group V compounds to form the desired III-V compound. The exact compound that is formed is controlled by the relative concentrations of the elements in the vapor phase. MOCVD deposition generally uses zinc (Zn), as the P-type dopant. However, it has been found that p-n junction definition can be difficult due to the Zn diffusion that occurs at typical MOCVD growth temperatures in excess of 600xc2x0 C. The second group of growth methods are those based on ultra-high vacuum deposition techniques of which the original method is known as Molecular Beam Epitaxy (MBE). In MBE heated elemental sources are used to produce an evaporated beam of atoms, or molecules, which are directed at a heated substrate. MBE deposition generally uses beryllium, as the P-type dopant. Beryllium has shown improved junction definition due to its low diffusivity at doping concentrations and typical MBE growth temperatures of around 500xc2x0 C. for InP substrate-based III-V alloys. Thus it is easier to define PN junctions in material that is grown with MBE methods. Both of these growth methods are well known in the art and it is therefore unnecessary to enter into a detailed description of them at this time.
A schematic diagram of a generic laser diode structure is shown in FIG. 1. There are three basic layers in overall structure 100. Layer 110 is known as the lower confining layer. This layer may be a binary, ternary or quaternary III-V compound and often has a thickness on the order of a micron. In this example this layer will be assumed to be InP that is doped n-type. Layer 120 forms the xe2x80x9cactivexe2x80x9d layer or the layer that produces light. In its simplest form layer 120 is a single layer however layer 120 will often comprise a plurality of sub-layers, as shown in FIG. 1. This latter structure forms a plurality of quantum wells or a Multi-Quantum Well (MQW) structure. A second, upper confining layer, layer 130, is formed on top of the active layer. This layer, like the lower confining layer is InP in this example and has a thickness on the order of a micron. In order to have a p-n junction structure the upper confining layer is doped P-type. The upper and lower confining layers have higher bandgaps and refractive indices than the active layer, so that both injected charge carriers and the light that is generated by their recombination are confined in the plane of the active layer. The final layer of the laser diode is a contact layer such that a low resistance contact is made to the structure. In the current example this layer is GaInAs, that is lattice matched to InP, and is between 0.2 and 0.5 xcexcm thick.
An area of considerable interest in fiber-optic based telecommunications are 1.3 xcexcm, 10 Gb/s lasers. One laser diode for this application comprises an AlGaInAs quaternary active layer and Indium Phosphide (InP) upper and lower confining layers. It is desirable to grow such Al containing III-V compounds with MBE methods as it is known in the art that it is difficult to grow Al containing compounds of the desired purity with MOCVD methods as Al containing precursors used for MOCVD growth are often contaminated with impurities. These impurities can lead to a degradation in the properties of the laser. Also growth of material with the desired quality may require growth at much higher temperatures i.e. greater than or approximately equal to 700xc2x0 C., which further complicates the production of accurately defined p-n junctions.
MBE methods are desirable for the formation of the Al containing quaternary used for the active layer. However MBE growth of P-type (Be doped) InP, used as the upper confining layer in an AlGaInAs based laser, is known to be susceptible to oxygen contamination. Oxygen may enter via a leaky reaction vessel in either MOCVD or MBE growth systems. However, the primary source of oxygen during MBE growth of InP has been found to be the phosphorus source. It is also known that the oxygen does not enter pure InP, rather it is incorporated into InP during the growth of beryllium doped InP, and to a lesser extent during the growth of Si-InP. It has been found that the presence of beryllium facilitates the incorporation of oxygen into the growing InP. Oxygen enters the lattice as a mid-gap donor level and compensates the holes associated with the beryllium acceptors thereby disrupting the P-type doping.
Moreover it would be desirable to use MBE methods to grow III-V semiconducting compounds that contain Al due to the difficulties in obtaining Al containing MOCVD precursors of adequate purity. However difficulties may arise due to the incorporation of oxygen during beryllium doping of an InP confining layer. Thus, there is a need for a method of doping III-V semiconducting compounds with beryllium that does not facilitate the incorporation of oxygen into the InP.
The invention is directed to a structure, and method of fabrication, for doping III-V compounds grown by MBE methods. In one embodiment the invention provides for beryllium doping of InP. The structure of the invention is a multi-layered structure in which layers comprising beryllium are deposited between layers of initially undoped InP. Beryllium diffuses from the layers comprising beryllium into the layers of initially undoped InP.
In accordance with one aspect of the invention a semiconducting structure that is comprised by a semiconducting device, whose major constituents include group III and group V elements, forming a III-V semiconducting compound, and whose minor constituents include dopant atoms is presented. The semiconducting structure comprises; a plurality of first layers, the first layers comprising a III-V compound and dopant atoms wherein the dopant atoms are distributed in a substantially uniform manner, and at least one second layer, the second layer comprising dopant atoms such that the concentration of dopant atoms in the second layer is larger than the concentration of dopant atoms in the first layer, and the second layer being located between two first layers.
In accordance with another aspect of the invention a semiconducting structure whose major constituents include indium and phosphorus, forming InP, and whose minor constituents include beryllium is presented. The semiconducting structure comprises; a plurality of first layers comprising InP, the first layers being doped with beryllium, and a plurality of second layers comprising beryllium, the second layers alternating with the first layers in the semiconducting structure wherein the beryllium of the second layers diffuses into the first layers to form a substantially uniform doping of the first layers.
In accordance with another aspect of the invention a method of fabricating a semiconducting structure whose major constituents include group III and group V elements, forming a III-V compound, and whose minor constituents include dopant atoms, the method of fabrication using molecular beam epitaxial methods is presented. The method comprises the steps of; (a) provision of the group V element of the III-V compound, (b) initiating deposition of the group III element of the III-V compound to form a first layer of the desired III-V compound, (c) terminating deposition of the group III element after the formation of a desired layer thickness of the first layer of III-V compound, (d) initiating deposition of the dopant to form a second layer, (e) terminating deposition of the dopant after the desired thickness of the second layer has been deposited, (f) initiating deposition of the group III element of the III-V compound to form a second layer of the desired III-V compound, (g) terminating deposition of the group III element after the formation of a desired layer thickness of the second layer of III-V compound, (h) determining if the required semiconductor structure has been formed, (i) returning to step (d) if the final semiconductor structure has not been formed, and (j) terminating deposition of the group V element after the desired semiconductor structure has been formed.
Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings.